The present invention relates to reducing the effects of interference from crosstalk and other noise sources that interfere with signals transmitted over wires, cable, fiber optics, wireless, or other types of communication where the signals suffer from some level of interference.
Interference generally is generated by signals from other signal sources. For example in Digital Subscriber Line (DSL) systems, interference may originate from other DSL services, in which case it is referred to as crosstalk, or from non-DSL sources such as AM radio transmitters, electrical appliances, and power supplies. Interference does not include unstructured background noise, such as thermal and environmental noise.
Interference in a signal may lead to certain limitations of a communication system. For example in wireless systems, such as cellular phones, interference may shorten the distance at which the signal can be reliably received and the clarity of the signal. As another example, in wireline systems, such as digital subscriber lines (DSL), interference may shorten the distance at which the signal can be reliably received, i.e., limit loop reach. Interference may also decrease the bit rate of the data being transferred. Providers of telecommunications services recognize the need to monitor the quality of service provided to users of their networks and to identify the causes of problems reported by their customers. This task, however, is complicated significantly by several factors.
The following discussion describes in detail many of the problems of DSL technology and potential solutions thereto. However, the discussion merely uses DSL as one example of the many communication systems (e.g., wireline, wireless, optical, cable, etc.) in which the present invention may be used. Thus the present invention should not be limited to merely DSL communication systems.
Digital Subscriber Line (DSL) networks provide high speed networking service while preserving the investment made in traditional telephone lines used for standard analog baseband telephone service, known as “plain old telephone service” or POTS. FIG. 1 shows an exemplary topology of a DSL network. In the exemplary DSL network topology 100 of FIG. 1, various customer premise equipment (CPE) modems 105, 106, 107 are communicatively coupled to a central office switching center 101 via ordinary telephone lines (e.g., lines 120 through 125).
Customer premise equipment 105, 106, 107 is equipment located at the customer's location (e.g., a customer's home or office). In the exemplary network topology 100 of FIG. 1, the customer premise equipment 105, 106, 107 possesses at least one transceiver (e.g., transceiver 108 in CPE 105) that is responsible for: 1) controlling at the CPE the reception of information sent from the service provider; and 2) controlling at the CPE the transmission of information sent to the service provider.
Information that flows in the network 100 toward the customer (e.g., toward the direction of a CPE as seen in FIG. 1) has a “downstream” direction while information that flows in the network 100 away from the customer (e.g., away from a CPE as seen in FIG. 1) has an “upstream” direction. Thus it may be said that a transceiver within a CPE is responsible for controlling at the CPE the transmission of upstream information and the reception of downstream information.
It will be appreciated that in the topology of FIG. 1, the customer premise equipment includes plural transceivers; such a topology may be applied where the bandwidth desired by a customer cannot be supplied by a single transceiver.
Various DSL service schemes exist. For example, at a high level, DSL services are characterized according to the bandwidth allocated for a customer's upstream and downstream traffic. Services that reserve approximately equal amounts of bandwidth for a customer's upstream and downstream traffic are referred to as “symmetric DSL” while services that reserve approximately unequal amounts of bandwidth for a customer's upstream and downstream traffic are referred to as “asymmetric DSL”.
ISDN DSL (IDSL), High bit-rate DSL (HDSL, HDSL2, HDSL4), Symmetric DSL (SDSL), and Single-pair High-speed DSL (SHDSL) are versions of symmetric DSL. Rate Adaptive DSL (RADSL), Asymmetric DSL (ADSL, ADSL2, ADSL2+), Splitterless ADSL (G.lite), and Very high bit rate DSL (VDSL) are versions of asymmetric DSL. Any of these DSL services (as well as other potential future DSL services that are not listed above) may be referred to as “DSL” or “xDSL”.
Note that the central office 101 includes a plurality of DSL Access Multiplexers 102, 103, 104 (DSLAMs). A DSLAM operates as a distributor of DSL services. That is, for example, DSLAM 102 forwards/collects downstream/upstream information sent from/to higher layers of a service provider's network to/from transceivers 108, 109, 110. The service provider's DSL network is controlled by a Network Management Agent (NMA) 118.
An NMA 118 is one or more software routines that monitor the operation of a network (e.g., by collecting various performance monitoring statistics sent from the DSLAMs 102, 103, 104) and controls various aspects of a network (e.g., by enabling or disabling service on a particular line). The NMA 118 shown in FIG. 1 monitors and controls the DSL network 100 by communicating with the DSLAMs through the Element Management Systems 116, 117 (EMSs). The NMA 118, as an example, may be executed as part of a network's Network Management System (NMS). An EMS effectively distributes to the DSLAMs control information sent from the NMA and forwards to the NMA 118 network performance or network status indicia sent from the DSLAMs. More details on a DSL system are provided below.
FIG. 2 shows a simplified depiction of an xDSL receiver Rx 201 within a DSL transceiver TRX 208. For example, transceiver 208 of FIG. 2 may be viewed as corresponding to transceiver 108 of FIG. 1 and line 220 of FIG. 2 may be viewed as corresponding to line 120 of FIG. 1. Recalling that the transceiver 208 is responsible for controlling both the transmission of upstream traffic and the reception of downstream traffic, note that receiver 201 assists the performance of the latter of these two functions.
The receiver 201 includes an equalizer 202 and a symbol detection unit 203 (which may also be referred to as a symbol detector 203). The equalizer 202 adjusts the transfer function of the receive channel such that the frequency components of the received waveform rx(t) 221 that are associated with the signal (i.e., the frequency components of the received waveform rx(t) 221 that are associated with the downstream information sent from the service provider to the transceiver 208) are enhanced with respect to the frequency components of the waveform rx(t) 221 that are not associated with the signal (i.e., the frequency components of the waveform's “noise”). It will be appreciated that the equalizer 202 may include several disparate blocks that collectively accomplish this function.
The symbol detection unit 203 converts the features of the equalized waveform 222 into digital 1's and 0's according to the modulation scheme employed by the particular type of DSL service being implemented. As a result of the equalizer's activity, the signal-to-noise ratio (SNR) in the receive channel is enhanced and the performance of the symbol detection unit 203 (i.e., its ability to correctly reproduce the digital information sent by the service provider) is improved. It will be appreciated that the symbol detection unit 203 may also include several disparate blocks that collectively accomplish this function.
Referring back to FIG. 1, note that the ordinary telephone lines such as 120, 121 and 122 that couple the DSLAMs and the CPEs are tightly packed together in a binder such as binder 114 and binder 115. Because ordinary telephone lines were originally designed for low speed voice/telephony communications, they are typically packed in a binder without shielding. That is, the lines typically comprise a simple twisted pair of wire that was originally intended to transmit only low-frequency voice signals. As a result, when used for high-frequency data transmission, these twisted pairs are not adequately protected from receiving high-frequency electromagnetic interference associated with the waveforms that appear on another line; nor are the waveforms on a line prevented from radiating so as to interfere with the waveforms that appear on another line.
For example, FIG. 3 illustrates a binder 308, which may be understood to correspond to the binders 114, 115 etc. shown in FIG. 1, having twisted pairs 306-1 through 306-N, which may be understood to correspond to the individual lines 120, 121 and 122 of FIG. 1. Pair 306-1 may be expected to experience more crosstalk from a pair 306-2 closer to it than more distant 306-L. Likewise, pair 306-2 located on the perimeter of the bundle 308 may experience different crosstalk than a pair 306-M more toward the center of the bundle 308. Additionally, if pair 306-1 had previously been the only pair being utilized for DSL service, but thereafter pair 306-2 was placed into DSL service, there may be new crosstalk due to this activation. Also the type of DSL service (i.e., ADSL, SHDSL, etc.) may have an effect on crosstalk. In general, each DSL service type occupies a band limited frequency region. If pairs in proximity to each other are conveying information in different frequency bands, then there may be less crosstalk between them than between pairs conveying information in the same frequency band.
Crosstalk may corrupt the operation of the symbol detection unit 203 discussed above with respect to FIG. 2. Crosstalk typically increases as the frequencies of the waveforms on an ordinary telephone line increase. Crosstalk is typically viewed as consisting of (1) Near-End Crosstalk (NEXT), caused by transmitters located at the “near-end”, i.e., on the same side (network side or customer side) of the connection as the receiver experiencing the interference, and (2) Far-End Crosstalk (FEXT), caused by transmitters located at the “far-end”, i.e., on the opposite side of the connection from the receiver experiencing the interference.
When the ordinary telephone lines were originally installed to carry POTS voice traffic, crosstalk was insubstantial because of the lower frequencies used to transmit voice traffic. In particular, the twisting of the two wires to form pairs such as the pairs 306 shown in FIG. 3, was designed to adequately protect against crosstalk interference from other twisted pairs at these low frequencies. However, as DSL is designed to provide higher speed services (as compared to traditional telephony service) over these ordinary telephone lines, DSL waveforms contain much higher frequencies, at which the twisting of the wires does not provide adequate protection. As a result, crosstalk from DSL waveforms is much more severe. The more severe crosstalk frequently hampers the successful deployment of a DSL service. The only way to substantially reduce this crosstalk is to replace the billions of existing copper pairs with shielded copper pairs, or other media such as optical fiber, which may not be economically feasible in many environments.
In view of the difficulties described above, there have been efforts to utilize signal processing techniques to improve communications in noisy and impaired channels such as DSL over telephone lines. Specifically, U.S. Pat. No. 6,970,415, assigned the assignee of this application and incorporated by reference herein, describes a method for characterizing and identifying crosstalk interference sources in a DSL or similar communication environment. U.S. Pat. No. 6,834,109, also assigned to the assignee of this application and incorporated by reference herein, describes a method for compensating for crosstalk interference by estimating and compensating for the presence of interfering signals. These methods may be implemented in a single DSL receiver to reduce the effect of crosstalk from other DSL services, and are therefore relevant even to residential DSL services which are typically delivered using a single copper pair.
Another way to increase the data throughput rates of services delivered to DSL customers is to use multiple copper pairs for a single customer. This process is called “bonding”, and it involves breaking up a data stream into multiple smaller data streams that are transmitted using multiple transmitters over multiple copper pairs, then received at the other end using multiple receivers, and finally reassembled back into the original larger data stream. Bonding of multiple lines, i.e., multiple copper pairs, is typically used to deliver services to business customers who often require higher data rates, especially in the upstream direction, than residential DSL customers. Such is illustrated in FIG. 1, as noted above.
When utilizing multiple lines for bonded services, it is possible to reduce the detrimental effects of crosstalk interference by coordinating the signals received in the receivers connected to those multiple lines. In that case, one may distinguish between two types of crosstalk:                1. “Self-crosstalk”, which consists of Self-NEXT and Self-FEXT, originates from transmitters connected to other lines of the same multiline connection, also referred to as “in-domain” transmitters and “in-domain” lines, respectively.        2. “Alien crosstalk”, which consists of Alien NEXT and Alien FEXT, originates from transmitters connected to lines that are not part of the same multiline connection, also referred to as “out-of-domain” transmitters and “out-of-domain” lines, respectively.        
Self-NEXT can be dealt with in a straightforward manner, because the signals of the interfering transmitters are known and thus their effect can be cancelled from the affected receivers. This cancellation is based on identifying the precise filter that has to be applied to the interfering transmitted signal to match the exact opposite of the interference signal at the affected receiver. The principle is the same as that used for echo cancellation in single-line transceivers, but in this case it involves additional complexities associated with a full matrix of crosstalk cancellation filters. The resulting “matrix echo canceller” contains filters that characterize the interference channels from each of the multiple near-end transmitters to each of the multiple near-end receivers.
The mitigation of Alien interference, which includes Alien crosstalk from out-of-domain transmitters, and also noise from non-DSL signal sources, is more challenging, since the in-domain receivers do not have access to the signals of the interfering out-of-domain transmitters or non-DSL signal sources. Nevertheless, addressing the problem of Alien interference is essential to achieving higher bitrates in a multiline system. Without it, Self-NEXT cancellation is of limited benefit, since there is no guarantee that the in-domain component of the interference will be stronger than the out-of-domain or non-DSL component.
Published PCT patent application WO 2003/105339, also assigned to the assignee of this application and incorporated by reference herein, describes a method for mitigating the effects of Alien interference by identifying its spectral signature matrix across multiple receivers, and then pre-whitening the received noise signal across these multiple receivers through the application of an appropriate pre-processing matrix to the outgoing signals prior to transmission, and of the corresponding post-processing matrix to the received signals before decoding them. In addition, the aforementioned patent application describes a method for eliminating Self-FEXT by using a matrix receiver filter that characterizes the transmission channels from each of the multiple far-end transmitters to each of the multiple near-end receivers. Thus, each of the multiple near-end receivers utilizes the signals from each of the multiple far-end transmitters, and the signal component that would ordinarily be considered Self-FEXT actually becomes part of the received signal.
The methods that have been disclosed in the aforementioned patents can be generally characterized as Multi-Input, Multi-Output (MIMO) processing techniques. In the case of PCT patent application WO 2003/105339, these techniques are applied to both the far-end transmitters and the near-end receivers in a system of multiple copper pairs impaired by noise and crosstalk. A disadvantage of using MIMO techniques on both transmitter and receiver is the need to establish special communication methods between the transmitter and receiver so that the MIMO processing at transmitter and receiver may be coordinated as the channel interference is modeled and managed. Specifically, the receiver must compute MIMO matrix parameters to optimize the overall channel transfer function, and then deliver those parameters to the transmitter to implement the MIMO system. A large number of parameters must be transmitted, and the additional channel of communication needed for delivery of these parameters may require approval by the appropriate DSL standards bodies.